1. The field of art to which the invention pertains includes the art of expansible chamber devices.
2. The use of guard devices for protecting pressure gauges or other forms of pressure responsive instruments such as indicators, recorders, transmitters, etc. for direct fluid contact of the pressure source are well known and are available from a variety of commercial sources. Typically, such guards are employed to prevent injury to the instrument from over-pressure, corrosion, freezing, sediment plugging or the like. Exemplifying guard devices of the prior art are those disclosed in U.S. Pat. Nos. 3,202,063 and 2,839,087.
In a common threaded type construction, guard devices of the proir art include a flexible diaphragm secured between bolt assembled upper and lower housings each defining a fluid passage communicating with the upper and lower faces, respectively, of the diaphragm. A fluid inlet in the lower housing is usually formed as a small diameter tapped opening for connecting a threaded pipe to the pressure source with which the instrument is to be utilized, while the outlet is similarly tapped for connection of the instrument. Flange type constructions are similarly available in which the diaphragm is secured between matched flanges that can also be used for matchup installation purposes.
To accommodate installation, such prior art threaded type devices are frequently sold as loose components assembled on site or alternatively assembled when sold but requiring disassembly during installation for making the threaded source connection to its inlet. By contrast, the flange type does not usually require disassembly except for removal of a flange face protector provided during shipment. However, because the lower mounting flange of such flange type constructions are "wetted", i. e. directly exposed to the pressured fluid, it has been customary to manufacture at least that flange of costly corrosion resistant metals. Consequently, guard devices of the prior art have been generally characterized as either subject to loose or misplaced parts causing inconvenient and even improper installation and/or of costly manufacture. Despite recognition of the foregoing problems, a ready solution has not heretofore been known.